1. Field of the Invention
This invention relates to a process for regenerating slag produced in steelmaking processes and utilizing the regenerated slag in further steelmaking operations while recovering the metal constituents produced by the regeneration. More particularly, the invention relates to cleansing molten slag of impurities and returning the purified slag to the refining furnace for utilization as a heat transfer agent and a fluxing material.
2. Description of the Prior Art
The refining of iron into steel is always accompanied by the production of a chemically complex by-product called "slag." Although steelmaking slags are sometimes used in blast furnace burden to recover the iron units present therein, such slags have been generally regarded as waste material and discarded because the cost of recovery of the constituents of the slag has been considered uneconomical.
Steelmaking slag is formed from the (i) slag-making materials purposely added to the iron charge to be refined, and (ii) impurities removed from the iron by oxidation. Steelmaking slags are classified as "acid" or "basic" according to chemistry of the process by which the iron is refined. If the iron to be refined contains such a small amount of phosphorus that need not be removed, a slag rich in silica is formed and is termed "acid." If, however, the phosphorus content of the iron is such that phosphorus removal is necessary, the chemistry requires formation of a "basic" slag. Consequently, a process in which the slag can remain acid throughout the process utilizes acid refractories in the furnace lining; whereas a process in which the slag is rendered basic requires the use of basic refractories -- otherwise the refractories would be attacked by the slag and would be shortlived.
By far, the predominant process used throughout the world in making steel is the basic process. Thus, the specific descriptions of steelmaking as they relate to the present invention will be cast in terms of the basic process, while the present invention should not be restricted thereto since it is also applicable to the other steelmaking processes.
Slag-making procedures in the basic process typically call for the maintenance in the slag of a ratio of total basic components to total acid components (called the "V-ratio") of about 3:1. The oxides (e.g., SiO.sub.2, P.sub.2 O.sub.5 Al.sub.2 0.sub.3, etc.) which form anion complexes in the melt (e.g., silicate, phosphate, aluminate, etc., anions) are said to be acidic oxides. The oxides (e.g., CaO, MgO, MnO, FeO, etc.) which break down the anion complexes in the melt are said to be basic oxides. The ratio of the concentrations of basic oxides to those of the acidic oxides is called the basicity of the slag. The common method of representing basicity is in terms of the V-ratio which, in simple slags, is the concentration ratio % CaO/% SiO.sub.2. In complex slags, the amounts of MgO present are added to the basic constituents and the amounts of P.sub.2 O.sub.5 to the acid constituents in computing the V-ratio.
In a typical basic steelmaking process, say the basic oxygen process, the chief impurities to be removed from the molten iron are silicon, manganese, carbon, phosphorus and sulfur. Removal of these impurities is accomplished by combining them with oxygen to form oxides. The carbon combines directly with oxygen and passes from the melt in gaseous form, either CO or CO.sub.2. Silicon and manganese form liquid oxides which become part of the slag. Phosphorus and sulfur combine with the materials in the slag, capable of forming compounds with them. After the slag has been used in the steelmaking process, it is contaminated with these oxides and no longer has utility in subsequent steelmaking operations. Further a substantial amount of the metals of which the contaminating oxides are based have value in unoxidized form, and a substantial portion of iron is also in the slag which must be discarded along with the other undesirable materials.
The value of the oxidized metal contaminants has been recognized by the steelmaking industry. A method of recovery of manganese from slag was discussed in an article in Steel magazine (Nov. 26, 1951, pp. 84-85). U.S. Pat. No. 3,002,832 (1961) discloses a process for recovering metallic iron by the reduction of slag with coke breeze. U.S. Pat. Nos. 3,761,247 and 3,717,490 are also illustrative of the steel industries interest in the purification of slag.
The sensible heat of the slag is wasted since it is conventionally disposed of in molten form. Because of the rising costs of fuels, the steelmaking industry is interested in methods of conserving energy and one such measure is the recovery of the sensible heat of molten spent slag.